Over the past several decades, the frequency of antimicrobial resistance and its association with serious infectious diseases have increased at alarming rates. The increasing prevalence of resistant pathogens, especially as agents causing nosocomial infections, also called hospital-acquired infections, is particularly disconcerting. Of the over 2 million nosocomial infections occurring each year in the United States, 50 to 60% are caused by antimicrobial-resistant strains of bacteria. The high rate of resistance to commonly used antibacterial agents increases the morbidity, mortality, and costs associated with nosocomial infections. In the United States, nosocomial infections are thought to contribute to or cause more than 77,000 deaths per year and cost approximately $5 to $10 billion annually. Among Gram-positive organisms, the most important resistant pathogens are methicillin-(oxacillin-) resistant Staphylococcus aureus (MRSA), β-lactam-resistant and multidrug-resistant pneumococci, and vancomycin-resistant enterococci. Important causes of Gram-negative resistance include extended-spectrum β-lactamases (ESBLs) in Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis, high-level third-generation cephalosporin (Amp C) β-lactamase resistance among Enterobacter species and Citrobacter freundii, and multidrug-resistance genes observed in Pseudomonas, Acinetobacter, and Stenotrophomonas. 
The problem of antibacterial resistance is compounded by the existence of bacterial strains resistant to multiple antibacterials. For example, Pseudomonas aeruginosa isolates resistant to fluoroquinolones are virtually all resistant to additional antibacterial medicines.
Thus there is a need for new antibacterials, particularly antibacterials with novel mechanisms of action or novel chemical structures that avoid at least some of the prevalent resistance mechanisms. Much of the antibacterial discovery effort in the pharmaceutical industry is aimed at the development of drugs effective against gram-positive bacteria. However, there is also a need for new Gram-negative antibacterials. Gram-negative bacteria are in general more resistant to a large number of antibacterials and chemotherapeutic agents than are gram-positive bacteria.